This invention relates to a lens array, an exposure device, an image forming apparatus and an LED head.
There is known an image forming apparatus such as a printer, a copier, a compound machine or the like using an array of LEDs (for example, an LED printer). Such an image forming apparatus uses an array of rod lenses for focusing the lights emitted by light emitting portions (i.e., the LEDs) onto a surface of a photosensitive drum.
The rod lens is formed of a glass fiber impregnated with ion so that the refractive index thereof decreases from the center portion toward the peripheral portion. The rod lenses functions as an optical element that forms an erected image of an object at the same magnification. The lens array in which the rod lenses are arranged in a plurality of rows is used as an optical system for focusing an image of an object as a linear image.
Recently, there is proposed another lens array including micro lenses disposed in the direction of the optical axis at an interval corresponding to the focal length (see, for example, Japanese Laid-Open Patent Publication No. 2000-221445). The micro lenses are also arranged in the direction perpendicular to the optical axis.
However, in the lens array using the micro lenses, it is necessary to provide a light-blocking portion for preventing the light from passing through adjacent micro lenses (i.e., the micro lenses that are not aligned with each other). Such a light-blocking portion needs to have openings arranged at an arrangement interval corresponding to an arrangement interval of the micro lenses. In the case where the arrangement interval of the openings is narrow, it becomes difficult to form the openings, and it becomes difficult to reduce the error of the arrangement interval of the openings. As a result, it becomes difficult to form the openings correctly in accordance with the positions of the micro lenses.